Determination of the H_3 Factor in Hydrogen Isotope Ratio Monitoring Mass Spectrometry

Abstract

The H_3 factor, K, is a parameter required in high precision, mass spectrometric analyses of hydrogen isotopic abundances. When H_2 is used as the sample gas, R* = R - Ki_2, where R* is the true HD/H_2 ratio, R is the observed (mass 3)/(mass 2) ion-current ratio, and i_2 is the ion current at mass 2. Four different methods for the determination of K were defined and tested under conditions characteristic of isotope ratio monitoring systems. Three of these were peak-based. The fourth employed steady flows of H_2 from a conventional inlet system. Results obtained using the latter method were more precise (standard deviation of K = 0.1 versus ~0.6 ppm mV^(-1) for the peak-based methods). However, use of the resulting values of K for correction of isotope ratio monitoring GC/MS results led to systematic errors as large as 9‰, whereas use of the peak-based values led to no systematic errors. Values of K were only weakly dependent on the pressure of He, declining ~5% for each 10-fold increase in P_(He). Small variations in partial pressures of H_2O and CH_4, potential contaminants under isotope ratio monitoring conditions, had no significant effect on values of K.

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